Make classes.jar the output of uninstallable Soong Java modules

The soong_java_prebuilt.mk file used to assume that if a Soong module
defines a dex jar, it is the output of the module and the module is
installable. This may now not be true if the module uses 'compile_dex'
to define a secondary build rule for a dex jar. If the module is not
installable (LOCAL_UNINSTALLABLE_MODULE=true), do not attempt to preopt
and copy classes.jar to the modules output destination.

Bug: 79409988
Test: on related CL
Change-Id: Ica958ac793e09a0e52125448b44e7cd36e7f35d5